- A valid Sudoku has exactly one solution — no solution or multiple solutions means the puzzle is flawed
- A "no solution" puzzle contains a logical contradiction in its given digits
- A "multiple solutions" puzzle lacks enough constraints to determine a unique answer
- 90%+ of apparent unsolvability is a solving error — verify your work first
- Puzzles from reputable sources are verified computationally and are always valid
Two Types of Invalid Puzzles
No-solution puzzles: The given digits contradict each other. For example, a row might already contain a digit twice in the given clues. If you are filling in correctly and reach a cell with zero valid candidates, either you made an error or the puzzle itself is contradictory.
Multiple-solution puzzles: The given digits are not enough to uniquely determine the solution. Two or more completed grids satisfy all constraints. This is the more common flaw in poorly made puzzles and is invisible until you reach the ambiguous region.
Is It You or the Puzzle?
Before concluding the puzzle is broken, apply this checklist:
- Verify every given digit: does any given row, column, or box already contain a repeated digit?
- Review your placements: for each digit you placed, can you reconstruct the logical proof?
- Check your candidate updates: were all candidates removed correctly after each placement?
If none of these reveal an error, you may have a genuinely flawed puzzle.
How to Confirm a No-Solution Puzzle
Find the cell with no valid candidates. Trace backward through every elimination that removed candidates from that cell. At least one of those eliminations must be unjustified — either it came from a wrong placement, or the given digits themselves create the conflict. If the conflict traces directly to two given digits in the same unit, the puzzle is definitively broken.
How to Confirm a Multiple-Solution Puzzle
Reach a bifurcation point. Assume one option and solve all the way to completion — does the grid complete cleanly with no contradictions? Then backtrack, assume the other option, and solve again. If both paths complete without contradiction, the puzzle has multiple solutions and is invalid.
Reliability of Published Puzzles
All puzzles on Sudoku.by are generated and verified by algorithm to have exactly one solution. Read how the generation process works. If you encounter an apparent unsolvable puzzle on the site, please report it — it is almost certainly a solving error, but we take every report seriously.